Siren (sound device)

A siren is a sound generating device. Sirens are intended to warn people, such as civil defense sirens, or the sirens on emergency service vehicles such as police vehicles, fire engines, and ambulances. Civil defense sirens are mounted in fixed locations and are used to warn of natural disasters, attacks, or civil threats. There are two general types: pneumatic and electronic.

The original version would yield sounds under water, suggesting a link with the sirens of Greek mythology.

Many fire sirens serve double duty as tornado or civil defense sirens, alerting an entire community of impending danger. Most fire sirens are either mounted on the roof of a fire station, or on a pole next to the fire station. Fire sirens can also be mounted near government buildings, on tall structures such as water towers, as well as in systems, where several sirens are distributed around a town for better sound coverage. Most fire sirens are single tone and mechanically driven by electric motors with a rotor attached to the shaft. Some newer sirens are electronically driven by speakers, though these are not as common.

Fire sirens are often called "fire whistles", "fire alarms", or "fire horns." Although there is no standard signaling of fire sirens, some utilize codes to inform firefighters of the location of the fire. Civil defense sirens pulling double duty as a fire siren often can produce an alternating "hi-lo" signal as the fire signal, or a slow wail (either an Adagio/Andante wavering tone or typically several long blasts) as to not confuse the public with the standard civil defense signals which are alert (known as All clear in UK; steady tone) and attack (known as Red warning in UK; fast wavering tone). Fire sirens are often blasted once a day at noon and are also called "noon sirens" or "noon whistles".

The device was invented by the Scottish natural-philosopher John Robison. It was improved and given its name by Charles Cagniard de la Tour.

Pneumatic
The pneumatic siren, which is a free aerophone, consists of a rotating disk with holes in it (called a siren disk or rotor), such that the material between the holes interrupt a flow air from fixed holes on the outside of the unit (called a stator). As the holes in the rotating disk alternately prevent and allow air to flow it results in alternating compressed and rarefied air pressure, i.e. sound. Such sirens can consume large amounts of energy.

Centrifugal-Type
The Centrifugal-type Siren, Sometimes also known as a Centrifugal Drum Siren and also simply a Mechanical siren, is a common design for certain mechanical sirens and is similar to a pneumatic siren; it has a fixed cylindrical part known as the stator that interrupts forced air from an specially designed rotating component. The said rotating component consists of a central column about which a set of vanes that form a spiral, or ribs, are positioned. The vanes and column are usually contained (usually as a single unit) within a hollow cylinder that is attached to a shaft and has equally sized slots (or ports) in it (called a chopper or rotor). As the rotor rotates, air enters the rotor near the center (and with the help of the vanes) moves perpendicularly from the shaft to the slotted cylindrical wall. As the slots in the rotating rotor alternately prevent and allow air to flow when it is rotating, it results in alternating compressed and rarefied air pressure, i.e. sound (by deflection and centrifugal force); A centrifugal-type siren can produce a sound for a given air volume with its intensity and pitch determined by the RPM of the rotor; Or if lacking the stator, can produce more pressure for a given air volume. This subsequently requires proper hearing protection when up close for bigger sirens due to the intensity of the sound. Some designs of Centrifugal-type sirens are usually boosted by forcing air from a positive displacement pump and/or blower (through a pipe network) to the rotor whether it lacks vanes or has them, i.e. as in a Supercharged Siren (such as the Federal Signal Thunderbolt) or the SoCal Edison Model 120. They are typically almost more efficient than the other mechanical siren designs.

Axial-Type
The Axial-Type Siren, is a rare design for sirens and is the most similar to a traditional pneumatic disk siren; it consists of a rotating disk with holes in it (called a siren disk or rotor), such that the material between the holes interrupt a flow air forced trough a fixed disk shaped stator. As the holes in the rotating disk alternately prevent and allow air to flow, it results in alternating compressed and rarefied air pressure, i.e. sound. Traditional disk sirens can consume large amounts of energy, so to combat this, vanes are added behind the siren disk, making it slightly resemble a cross between an axial fan and a disk siren when viewed from behind or the intake side. They are typically less efficient and/or noisier than comparable centrifugal and disk type sirens, although some types of axial sirens are louder due to specially engineered vanes.

Hybrid-Designs


Most hybrid mechanical siren designs are super rare and almost don't exist anymore, but there are a few notable designs that stand out from the rest of them.

Some sirens including the E.D Bullard Manufacturing Company's sirens utilized a conical shaped rotor/stator assembly which was essentially a hybrid between the disk/axial siren and the common centrifugal drum siren.

Electronic
Electronic sirens incorporate circuits such as oscillators, modulators, and amplifiers to synthesize a selected siren tone (wail, sawtooth, bellring, or beebaw) which is played through external speakers.

Electronic sirens seem to be better for clearing traffic in front of vehicles, while pneumatic and mechanical sirens seem better at protecting vehicles passing or turning through an intersection. It is not unusual, especially in the case of modern fire engines, to see an emergency vehicle equipped with both types of sirens.